The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface of the substrate is non-planar, then a photoresist layer placed thereon is also non-planar. A photoresist layer is typically patterned by a photolithographic apparatus that focuses a light image onto the photoresist. If the outer surface of the substrate is sufficiently non-planar, then the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus, and it will be impossible to properly focus the light image onto the outer substrate surface.
It may be prohibitively expensive to design new photolithographic devices having an improved depth of focus. In addition, as the feature size used in integrated circuits becomes smaller, shorter wavelengths of light must be used, resulting in a further reduction of the available depth of focus. Therefore, there is a need to periodically planarize the substrate surface to provide a substantially planar layer surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted to a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier provides a controllable load, i.e., pressure, on the substrate to press it against the polishing pad. In addition, the carrier may rotate to provide additional motion between the substrate and polishing pad. A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be distributed over the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate.
A CMP process is fairly complex, and differs from simple wet sanding. In a CMP process, the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and the abrasive particles with the reactive sites results in polishing.
An effective CMP process should have a high polishing rate and generate a substrate surface that is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing rate sets the maximum throughput of the polishing apparatus.
The polishing rate depends upon the force with which the substrate is pressed against the pad. Specifically, the greater this force, the higher the polishing rate. If the carrier head applies a non-uniform load, i.e., if the carrier head applies more force to one region of the substrate than to another, then the high pressure regions will be polished faster than the low pressure regions. Therefore, a non-uniform load may result in non-uniform polishing of the substrate.
One problem that has been encountered in CMP is that the edge of the substrate is often polished at a different rate (usually faster, but occationally slower) than the center of the substrate. This problem, termed the "edge effect", may occur even if the load is uniformly applied to the substrate. The edge effect typically occurs in the perimeter portion, e.g., the outermost five to ten millimeters, of the substrate. The edge effect reduces the overall flatness of the substrate, makes the perimeter portion of the substrate unsuitable for use in integrated circuits, and decreases yied.
Therefore, there is a need for a CMP apparatus that optimizes polishing throughput while providing the desired flatness and finish. Specifically, the CMP apparatus should have a carrier head which provides substantially uniform polishing of a substrate.